The present invention is related generally to radio frequency (RF) signal filters, and more particularly to improved ceramic bandstop filters that are particularly well adapted for use in radio transmitting and receiving circuitry.
Conventional multi-resonator coaxial filters include a plurality of resonators that are typically foreshortened short-circuited quarter-wavelength coaxial transmission lines. The coaxial resonators may be inductively coupled one to another by apertures in their common walls. Each resonator can be tuned by means of a tuning screw which inserts into a hole extending through the middle of the resonator. Once tuned, the overall response of coaxial filters is determined by the size of the interstage coupling apertures. Since the tuning of coaxial filters can be disturbed by a slight adjustment of the tuning screw, a lock nut is required to keep the tuning screw properly positioned at all times. The use of tuning screws not only renders coaxial filters susceptible to becoming de-tuned, but also creates additional problems including mechanical locking of the tuning screw and arcing between the tuning screw and the resonator structure. Furthermore, coaxial filters tend to be rather bulky, and therefore are relatively unattractive for applications where size is an important factor.